System and method for controlling linear pump system

ABSTRACT

Systems and methods for operating a linear pump system involve operating a linear motor system; reciprocating a linear pump and a motor control module that issues commands and a control logic input to the linear motor system. The linear motor system is operated to reciprocate an output shaft between first and second reversal positions. The linear pump is reciprocated with the output shaft to produce a flow of material. A pump reversal command reverses direction of the output shaft. A torque command controls speed of the output shaft. The control logic input reciprocates the output shaft at speeds to produce a constant output condition of the flow of material. The motor control module adjusts the torque command to operate the output shaft at an increased speed above what is necessary for the constant output condition for a temporary time period beginning when the reversal command is issued.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. §120 to U.S. provisional application Ser. No. 61/267,509, entitled “MINIMIZING PRESSURE DROP DURING PUMP REVERSAL IN A LINEAR PUMP SYSTEM,” filed Dec. 8, 2009 by inventors Christopher Blackson, Nick Long and Roger Blough, the contents of which are incorporated by this reference.

This application claims priority under 35 U.S.C. §119 to PCT application Serial No. PCT/2010/______, entitled “SYSTEM AND METHOD FOR CONTROLLING LINEAR PUMP SYSTEM,” filed Dec. 8, 2010 by inventors Christopher Blackson, Nick Long and Roger Blough, the contents of which are incorporated by this reference.

BACKGROUND

The present invention relates generally to pump control systems. More particularly, the present invention relates to reducing pressure drop in linear pumps.

Linear pumps include a piston that reciprocates in a housing to push fluid through the housing. Conventional linear pumps draw fluid into the housing on a backward stroke and push the fluid out of the housing on a forward stroke. Valves are used to prevent backflow through the pump. The valves can also be configured to draw in fluid and pump fluid on opposite sides of the piston during each of the backward stroke and forward stroke in order to provide a steady flow of fluid from the pump. There is, however, an inherent drop in pump pressure when the piston reverses direction, which results in variation of the volume of dispensed fluid. This variation is particularly undesirable when precisely metered flow is needed. In dual component metering systems, for example, a resin material and a catalyst material are simultaneously discharged from a mixing head of a dispensing gun. Mixing of the two materials produces a chemical reaction that begins a solidification process resulting in a hardened material after full curing. It is advantageous to provide even flow of the resin material and the catalyst material throughout the dispensing process to ensure a proper ratio of resin to catalyst such that the mixture properly cures. There is, therefore, a need for reducing the pressure loss associated with linear pumps used in single and dual component metering systems.

SUMMARY

The present invention is directed to methods and systems for operating a linear pump system.

A method of operating a linear pump system comprises operating a linear motor system and reciprocating a linear pump. The linear motor system is operated to reciprocate an output shaft between first and second reversal positions. The linear pump is reciprocated with the linear motor system output shaft to produce a flow of material. The linear motor system is driven at rates to provide a constant material flow output condition. The linear motor system is temporarily driven at an increased rate above what is necessary for the constant output condition when the output shaft reverses direction.

A linear pump system comprises a linear motor system, a linear material pump and a motor control module. The linear motor system produces a reciprocating movement of an output shaft between first and second reversal positions. The linear material pump is connected to the output shaft to produce an output flow of material. The motor controller issues a reversal command, a torque command and a control logic input to the linear motor system. The reversal command is issued to reverse direction of the output shaft. The torque command is issued to control speed of the output shaft. The control logic input is issued to reciprocate the output shaft at speeds to produce a constant output condition of the flow of material. The motor control module adjusts the torque command to operate the output shaft at an increased speed above what is necessary for the constant output condition for a temporary time period beginning when the reversal command is issued.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a dual-component pump system having a pumping unit, component material containers and a dispensing gun.

FIG. 2 shows a schematic of the dual-component pump system of FIG. 1 having individually controlled linear component pumps.

FIG. 3 shows a flow chart for a method of controlling a linear component pump of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows dual-component pump system 10 having pumping unit 12, component material containers 14A and 14B and dispensing gun 16. Pumping unit 12 comprises hydraulic power pack 18, display module 20, fluid manifold 22, first linear pump 24A, second linear pump 24B, hydraulic fluid reservoir 26 and power distribution box 28. As shown in FIG. 2, an electric motor, a dual output reversing valve, a hydraulic linear motor, a gear pump and a motor control module (MCM) for each of linear pumps 24A and 24B are located within hydraulic power pack 18. Dispensing gun 16 includes dispense head 32 and is connected to first linear pump 24A and second linear pump 24B by hoses 34A and 34B, respectively. Hoses 36A and 36B connect feed pumps 38A and 38B to linear pumps 24A and 24B, respectively. Compressed air is supplied to feed pumps 38A and 38B and dispensing gun 16 through hoses 40A, 40B and 40C, respectively.

Component material containers 14A and 14B comprise drums of first and second viscous materials that, upon mixing, form a hardened structure. For example, a first component comprising a resin material, such as a polyester resin or a vinyl ester, is stored in component material container 14A, and a second component comprising a catalyst material that causes the resin material to harden, such as Methyl Ethyl Ketone Peroxide (MEKP), is stored in component material container 14B. Electrical power is supplied to power distribution box 28, which then distributes power to various components of dual-component system 10, such as the MCM within hydraulic power pack 18 and display module 20. Compressed air from a separate source (not shown) is supplied to feed pumps 36A and 36B through hoses 40A and 40B to supply flows of the first and second component materials to linear pumps 24A and 24B, respectively. Linear pumps 24A and 24B are hydraulically operated by the gear pump in hydraulic power pack 18. The gear pump is operated by the electric motor in power pack 18 to draw hydraulic fluid from hydraulic fluid reservoir 26 and provide pressurized hydraulic fluid flow to the dual output reversing valve, which operates the linear motor, as will be discussed in greater detail with reference to FIG. 2.

When a user operates dispense gun 16, pressurized component materials supplied to manifold 22 by linear pump 24A and linear pump 24B are forced to mixing head 32. Mixing head 32 blends the first and second component materials to begin the solidification process, which completes when the mixed component materials are sprayed into a mold, for example. The first and second component materials are typically dispensed from gun 16 at a constant output condition. For example, a user can provide an input at display module 20 to control the MCM to dispense the component materials at a constant pressure or at a constant flow rate. The MCM uses control logic inputs and outputs in conjunction with the electric motor and the dual output reversing valve, among other components, to provide the constant output condition. However, because linear pumps 24A and linear pump 24B include pistons that must reverse direction, there is a slight variation in the constant output condition as pressures in the pumps drop at the reversal point. The present invention provides a control system and method for reducing variations in the pressures of the dispensed component materials that arise from pressure drops produced at reversal positions of linear pump 24A and linear pump 24B.

FIG. 2 shows a schematic of dual-component pump system 10 of FIG. 1 having individually controlled linear component pumps 24A and 24B. Pump system 10 includes pumping unit 12, dispensing gun 16, first linear pump 24A, second linear pump 24B, hydraulic fluid reservoir 26A, second hydraulic fluid reservoir 26B, motor control modules (MCMs) 42A and 42B, electric motors 44A and 44B, gear pumps 46A and 46B, dual output reversing valves 48A and 48B, hydraulic linear motors 50A and 50B, output pressure sensors 52A and 52B and velocity linear position sensors 54A and 54B. Hydraulic reservoirs 26A and 26B also include pressure relief valves 56A and 56B, filters 58A and 58B, level indicators 60A and 60B, and pressure sensors 62A and 62B, respectively.

Hydraulic fluid reservoir 26A, MCM 42A, electric motor 44A, gear pump 46A, dual output reversing valve 48A and hydraulic linear motor 50A are located within hydraulic power pack 18 and comprise first linear motor system 64A. Likewise, hydraulic fluid reservoir 26B, MCM 42B, electric motor 44B, gear pump 46B, dual output reversing valve 48B and hydraulic linear motor 50B are located within hydraulic power pack 18 and comprise second linear motor system 64B. In other embodiments of the invention, the linear motor systems share components, such as an electric motor, gear pump and hydraulic fluid reservoir.

With pumping unit primed and activated, pressurized first and second component materials are provided to linear pumps 24A and 24B, respectively by feed pumps 38A and 38B (shown in FIG. 1), respectively. Feed pumps 38A and 38B are operated with pressurized air. Linear pumps 24A and 24B are operated by first and second linear motor systems 64A and 64B to provide pressurized first and second component materials to dispensing gun 16. Also, pressurized air is provided to dispensing gun 16 to operate a pump or valve mechanism to release the pressurized component materials into mix head 32 and out of gun 16.

Linear motor systems 64A and 64B are controlled by motor control modules (MCM) 42A and 42B, respectively. MCMs 42A and 42B operate linear motor systems 64A and 64B in equal and identical manners so that proportional amounts of component material are provided to dispensing gun 16. Description of the operation linear motor systems 64A and 64B will be directed to linear motor system 64A, with operation of linear motor system 64B operating in a like manner, with like components being numbered accordingly.

Electric motor 44A receives electric power from power distribution box 28 (FIG. 1). In one embodiment, electric motor 44A comprises a direct current (DC) motor. MCM 42A issues torque command C_(T), which is received by motor 44A to control the speed of drive shaft 66A. Drive shaft 66A is coupled to gear pump 46A, which is submerged in hydraulic fluid within hydraulic fluid reservoir 26A. Gear pump 46A utilizes the rotary input from motor 44A to drawn in fluid from reservoir 26A and produce a flow of pressurized hydraulic fluid in line 68A. Hydraulic fluid reservoir 26A includes level indicator 60A, which is used to determine the amount of fluid within reservoir 26A. Pressure sensor 62A can be used to determine under-fill conditions within reservoir 26A. In other embodiments, drive shaft 66A is used to drive other types of positive displacement pumps that convert rotary input into pressurized fluid flow, such as rotary vane pumps or peristaltic pumps.

Pressurized hydraulic fluid from pump 46A flows past pressure relief valve 56A and to dual output reversing valve 48A. Relief valve 56A provides a means for allowing excess pressurized hydraulic fluid to return to reservoir 26A when excessive pressure conditions exists. As will be discussed below, reversing valve 48A uses the pressurized hydraulic fluid to reciprocate linear motor 50A. Pressurized hydraulic fluid returns to reservoir 26A from reversing valve 48A in line 70A after passing through filter 58A. Filter 58A removes impurities from the hydraulic fluid. Thus, a closed circuit flow of hydraulic fluid is formed between reservoir 26A, gear pump 46A, reversing valve 48A and linear motor 50A.

Dual output reversing valve 48A is constructed according to conventional reversing valve designs, as are known in the art. Dual output reversing valve 48A receives a continuous flow of pressurized hydraulic fluid and diverts the flow of fluid to linear motor 50A. Specifically, reversing valve 48A includes an input connected to line 68A, an output connected to line 70A and two ports connected to lines 72A and 74A. Pressurized fluid is alternately supplied to lines 72A and 74A, which is used to actuate linear motor 50A.

Linear motor 50A includes piston 76A, which slides within housing 78A between two fluid chambers. Each fluid chamber receives a flow of pressurized fluid from lines 72A and 72B, respectively. For example, with reversing valve 48A in a first position, line 72A provides pressurized fluid to a first chamber in housing 78A to move piston 76A downward (with respect to FIG. 2). Simultaneously, fluid within the other chamber in housing 78A is pushed out of linear motor 50A and back into reversing valve 48A through line 74A and out to line 70A. MCM 42A issues reverse command C_(R), which is received by reversing valve 48A to control when linear motor 50A begins reversing direction. After reverse command C_(R) is received, reversing valve 48A switches to a second position such that pressurized fluid is supplied to housing 78A through line 74A and fluid from housing 78A is removed through line 72A. Thus, operation of reversing valve 48A reciprocates piston 76A within housing 78A between two reversal positions, which also reciprocates output shaft 80A. Velocity linear position sensor 54A is coupled to shaft 80A and provides MCM 42A an indication of the position and speed of piston 76A based on the rate at which piston 76A is moving. In particular, position sensor 54A provides position signal S_(Po) to MCM 42A when output shaft 80A is moving away from one of the reversal positions.

Output shaft 80A of linear motor 50A is directly mechanically coupled to piston shaft 82A of linear pump 24A. Shaft 82A drives piston 84A within housing 86A. Piston 84A draws into housing 86A a component material from container 14A, as provided by feed pump 38A (FIG. 1). Linear pump 24A comprises a double action pump in which component material is pushed into line 88A on an up stroke (with reference to FIG. 2) and pushed into line 89A on a down stroke (with reference to FIG. 2). Specifically, on an up stroke, valve 90A opens to draw component material from feed pump 38A through manifold 22 (shown in FIG. 1) and into housing 86A, and valve 92A opens to allow piston 84A to push material into dispensing gun 16 through line 88A, while valves 94A and 96A are closed. On a down stroke, valves 90A and 92A close, while valve 94A opens to draw component material from feed pump 38A through manifold 22 (shown in FIG. 1) and into housing 86A, and valve 96A opens to allow piston 84A to push material into dispensing gun 16 through line 89A. The dual action of linear pump 24A maintains a continuous and near constant supply of component material during operation. As mentioned, however, at the reversal point of piston shaft 82A a slight pressure drop occurs. The present invention alleviates some of the experienced pressure drop by accelerating piston shaft 82A near the reversal point.

Component material from lines 88A 89A is pushed into dispensing gun 16 by pressure from linear pump 24A, where it mixes with component material from linear pump 24B within mix head 32 before being sprayed from gun 16. Pressure sensor 52A senses pressure of the component material within line 88A and sends pressure signal S_(Pr) to MCM 42A. Optional heater 98A can be attached to line 88A to heat the component material before dispensing from mix head 32 to, for example, reduce the viscosity of the component material or to facilitate reacting and curing with the other component material.

MCM 42A receives position signal S_(P)o and pressure signal S_(Pr) and issues reverse command C_(R) and torque command C_(T). Using position signal S_(P)o and pressure signal S_(Pr), MCM 42A coordinates reverse command C_(R) and torque command C_(T) to control linear motor system at a constant output condition. For example, an operator of dual-component pump system 10 can specify at an input in display module 20 (FIG. 1) that pumping unit 12 will operate to provide a constant pressure of the first and second component materials to manifold 22 (omitted from FIG. 2, shown in FIG. 1) or a constant flow output of the component materials to manifold 22. MCM 42A operates control logic that continuously adjusts reverse command C_(R) and torque command C_(T) to maintain the constant output condition. Torque command C_(T) determines how fast motor 44A rotates shaft 66A, which directly relates to how fast the chambers within housing 78A of linear motor 50A will fill with fluid. Reverse command C_(R) determines when reversing valve 48A switches position. Issuance of reverse command C_(R) is coordinated with how fast the chambers within housing 78A fill so that reversing valve 48A can switch the direction of fluid flow into housing 78A. The control logic maintains the speed of motor 44A and the switching rate of reversing valve 48A in concert to maintain the desired constant output condition. As will be discussed with reference to FIG. 3, the present invention operates linear motor system 64A to minimize pressure drops when piston shaft 82A reverses direction.

FIG. 3 shows a flow chart diagramming instructions for a method of controlling linear component pumps 24A and 24B of FIG. 2. At first step 100, motor control module (MCM) 42A operates linear pump system 64A using control logic inputs, e.g. programmed inputs or inputs entered at display module 20, such that linear pump 24A is operated at a constant output condition. Specifically, MCM 42A adjusts torque command T_(C) to electric motor 44A to control the speed of shaft 66A.

When pump shaft 82A gets to the end of its travel at a reversing point, MCM 42A issues control logic output in the form of reverse command C_(R) to reversing valve 48A as part of the control logic at step 110. Next, at step 120, MCM 42A turns off the control logic such that MCM 42A is no longer continuously updating torque command C_(T) and reverse command C_(R) to produce a constant output condition. At step 130, MCM 42A issues torque command C_(T) to motor 44A to increase the speed of shaft 66A, thereby increasing the output of pressurized fluid flow to dual output reversing valve 48A. Correspondingly, MCM 42A issues reverse commands C_(R) to reversing valve 48A to reverse the direction of pump output shaft 80A commensurate with the flow of pressurized hydraulic fluid. Specifically, reversing valve 48A must be operated to allow the chambers within housing 78A in linear motor 50A to be filled and evacuated at a flow rate equal to that provided by pump 46A. As such, after step 130 is performed, velocity of output shaft 80A is momentarily increased beyond what was previously being performed at step 100 to achieve the constant output condition. Output shaft 80A thus operates shaft 82A of linear pump 24A at an increased rate to reduce pressure drop in linear pump 24A when shaft 82A is at a reversal position within housing 86A. Output shaft 80A and piston shaft 82A thereby reverse direction more quickly than what would have occurred under the control logic regime.

At step 140, the control logic detects from linear position sensor 54A a reversal in direction of output shaft 80A. Subsequently, at step 150 torque command C_(T) to motor 44A is reduced below the levels instructed at step 130 by altering torque command C_(T). Typically, the speed is maintained some amount higher than what was being commanded under the control logic. The speed can, however, be reduced below speeds at step 100 in order to minimally disrupt the constant output condition if needed. At step 160, a reset condition is detected such that the control logic can be turned on at step 170. For example, a reset condition can be determined when the pressure in line 88A is sensed as increasing, indicating that shaft 82A and shaft 78A have completed the reversal process. Step 170 may occur immediately after a predetermined period of time, typically a few tens of milli-seconds so as to minimize disruption of the constant output condition. Thus, when MCM 42A receives pressure signal S_(Pr) from pressure sensor 52A, the control logic can be reestablished such that motor 44A and reversing valve 48A are again operated under the constant output condition, as is done at step 100. In other embodiments, the reset condition can be established based on a predetermined amount of time, such as a predicted time it takes for linear motor 50A to complete a reversal process based on the pump speeds of steps 130 and 150.

The present invention provides a system and method for reducing pressure variations during operation of a linear pump system. As discussed above, linear pumps inherently produce a reduction in output pressure when the piston reverses direction. With respect to the disclosed embodiment, the output pressure of the pump is directly proportional to the speed at which the piston moves, which is determined by the speed at which an electric motor drives the linear motor actuating the linear pump. Thus, when the piston stops to reverse direction, the pump pressure drops. The drop in pump pressure is particularly disadvantageous when constant output conditions are desired. Furthermore, under constant output conditions, input speeds to the electric motor driving the linear motor remain generally constant such that, even under constant pressure output conditions, the pressure output of the linear pump exhibits a slight waveform pattern. In one embodiment of the present invention, the speed of the electric motor is momentarily increased near the reversal point of the linear motor over the speed necessary to provide the constant output condition. As such, the slight waveform pattern of the output pressure is reduced.

Although the present invention has been described with respect to controlling a linear pump system based on speed or torque control of electric motors, the invention can be applied to other types of linear pump systems or can be used to control the described system in other ways. Rather than controlling the speed of the electric motor, the hydraulic fluid pressure can be controlled using a programmable hydraulic pressure regulator that controls speed of the linear motor in place of reversing valve 48A. Specifically, the pressure regulator would be fed with pressurized hydraulic fluid from the electric motor and gear pump combination and would feed fluid to either the linear motor or back to the fluid reservoir. The pressure regulator would then be programmed or controlled to alter the ratio of hydraulic fluid entering the linear motor and being fed back to the reservoir. Normally, control logic would be used to control a constant output condition with the pressure regulator. However, the control logic could be temporarily suspended and the output of the pressure regulator to the liner motor would be increased when the linear motor is at or near the reversal position. Alternatively, the hydraulic system and hydraulic pressure regulator of the described embodiment could be replaced with a pneumatic system and a pneumatic pressure regulator.

While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A linear pump system comprising: a linear motor system that produces a reciprocating movement of an output shaft between first and second reversal positions; a linear material pump connected to the output shaft to produce a flow of material; and a motor control module that: issues a reversal command to the linear motor system to reverse direction of the output shaft; issues a torque command to the linear motor system to control speed of the output shaft; and issues a control logic input to the linear motor system to reciprocate the output shaft at speeds to produce a constant output condition of the flow of material; wherein the motor control module adjusts the torque command to operate the output shaft at an increased speed above what is necessary for the constant output condition for a temporary time period beginning when the reversal command is issued.
 2. The linear pump system of claim 1 wherein the linear motor system further comprises: a linear hydraulic motor that includes the output shaft; an electric motor that receives the torque command from the motor control module; a rotary hydraulic pump driven by the electric motor that provides a flow of pressurized hydraulic fluid commensurate with speed of the electric motor to control speed of the output shaft; and a hydraulic dual output reversing valve coupled to the linear hydraulic motor and that receives the flow of pressurized hydraulic fluid from the rotary hydraulic pump to hydraulically reverse direction of the output shaft in response to receiving the reversal command from the motor control module.
 3. The linear pump system of claim 1 wherein the motor control module turns off the control logic input in response to adjusting the torque command and while operating the output shaft at speeds above what is necessary for the constant output condition.
 4. The linear pump system of claim 3 wherein the linear motor system further comprises: a position sensor connected to the linear hydraulic motor that issues a position signal in response to the output shaft moving away from one of the reversal positions; wherein the motor control module alters the adjusted torque command to reduce the increased speed of the output shaft in response to receiving the position signal.
 5. The linear pump system of claim 3 wherein the motor control module turns on the control logic input after the temporary time period terminates.
 6. The linear pump system of claim 5 wherein the temporary time period terminates when a reset condition is detected.
 7. The linear pump system of claim 5 wherein the linear motor system further comprises: a pressure sensor for sensing pressure in the flow of material; wherein the reset condition comprises a sensed pressure rise.
 8. The linear pump system of claim 1 wherein the constant output condition comprises either constant pressure output or constant flow output.
 9. A method for operating a linear pump system comprising: operating a linear motor system to reciprocate an output shaft between first and second reversal positions; reciprocating a linear pump with the linear motor system output shaft to produce a flow of a material; driving the linear motor system at rates to provide a constant output condition of the flow of material; initiating a reversal in direction of the output shaft; and driving the linear motor system at an increased rate above what is necessary for the constant output condition for a temporary time period beginning when the reversal in direction of the output shaft is initiated.
 10. The method for operating a linear pump system of claim 9 and further comprising: operating a linear hydraulic motor having the output shaft; operating an electric motor; operating a rotary hydraulic pump with the electric motor to provide a flow of pressurized hydraulic fluid to the linear hydraulic motor to control speed of the output shaft; and operating a hydraulic dual output reversing valve coupled to the linear hydraulic motor and that receives the flow of pressurized hydraulic fluid to reverse direction of the output shaft.
 11. The method of operating a linear pump system of claim 10 and further comprising: a motor control module that coordinates operation of the electric motor and the hydraulic dual output valve, wherein: the linear motor system is driven at rates to provide the constant output condition in response to receiving a control logic input from the motor control module; the reversal in direction of the output shaft is initiated in response to the hydraulic dual output valve receiving a reversal command from the motor control module; and the linear motor system is driven at the increased rate in response to the electric motor receiving a torque command from the motor control module.
 12. The method of operating a linear pump system of claim 11 and further comprising: turning off the control logic input when the torque command is issued while operating at the increased rate.
 13. The method of operating a linear pump system of claim 12 and further comprising: detecting a position of the output shaft at a reversal position; and altering the torque command to reduce the increased rate in response to detecting the reversal position.
 14. The method of operating a linear pump system of claim 12 and further comprising: turning on the control logic input after the temporary time period terminates.
 15. The method of operating a linear pump system of claim 14 and further comprising: sensing a pressure of the output flow material; and terminating the temporary time period when a pressure rise is sensed.
 16. The method of operating a linear pump system of claim 9 wherein the constant output condition comprises either constant pressure output or constant flow output.
 17. A method of operating a linear pump system comprising: operating a linear motor system to reciprocate an output shaft between first . and second reversal positions; reciprocating a linear pump with the linear motor system output shaft to produce a flow of a material; driving the linear motor system at rates to provide a constant output condition of the flow of material; and temporarily driving the linear motor system at an increased rate above what is necessary for the constant output condition when the output shaft reverses direction.
 18. The method of claim 17 wherein the step of driving the linear pump comprises: reversing direction of the output shaft; controlling speed of the output shaft; and wherein the direction and speed of the output shaft are controlled with control logic to provide the constant output condition, the control logic being suspended when the linear pump is temporarily driven at the increased rate.
 19. The method of claim 18 and further comprising: detecting a position of the output shaft; and reducing the increased rate when the output shaft is detected moving away from a reversal position.
 20. The method of claim 19 and further comprising: sensing a pressure in the flow of material; and initiating the control logic when a pressure rise is sensed. 